The present invention relates generally to methods and apparatus for the removal of gas borne solids in a thermal treatment process by the use of acoustic forces. More particularly, the invention is directed to the use of the acoustic force known as the Oseen force for the removal of particles from a high temperature flue gas.
A large variety of industrial power plant and environmental clean-up purposes are served by high temperature processing of fuels and contaminants, respectively. A common malady of these high temperature processes is the production of a hot gas or vapor flow with an undesirably high particulate content. A large number of Superfund applications require remediation of contaminated soil. Where these applications require handling of soils, as in excavation, conveyance, blending, thermal treatment, or deposition, engineers are faced with the problem of control of particulate emissions. Current stringent regulations place a ceiling on the level of particulate emissions which will be tolerated from the likes of coal-fired power plant flues or environmental clean-up emissions. Furthermore, it is desirable to remove particulates from a high temperature gas flow where such particulates may have an abrasive effect on downstream equipment. For example, the flue gas from a coal-fired boiler contains sulfur oxide gases, nitrogen oxide gases, and fly ash particulates which must be destroyed or removed before the clean gas is released into the atmosphere. Preferably the treatment is completed while the flue gas is at a high temperature such that it can spin a turbine or preheat air, without the abrasive effect of fly ash or the corrosive effect of acid anhydride gases.
High temperatures are also desirable in an application such as the remediation by an evaporation process of contaminated soil containing, for example, mercury. Removal of the volatile component or contaminant such as mercury by such an evaporation process must be carried out at a temperature above the dew point for the volatile substance, to prevent its condensation on the fly ash. For example, in a flue gas containing a mixture of mercury vapor, air, other vapors, and fly ash, the separation of the fly ash from the gas should take place near 400.degree. C. to prevent condensation of mercury on the particulates. For a clay soil, many of the particulates in the fly ash will be below 1 micrometer in diameter.
As another example, mixed wastes, consisting of materials contaminated with both radioactive constituents and hazardous non-radioactive constituents, are desirably treated at high temperature. This is because it is highly desirable to separate radioactively contaminated constituents from hazardous non-radioactive constituents since there is no available disposal means for mixed wastes, whereas radioactive wastes and hazardous wastes can be individually managed through currently available and approved methods. In a mixed waste process, gas flow will entrain solid particulates carrying radioactive contaminants. To prevent recontamination of these particulates by gas-entrained hazardous condensables, a separation must be effected at temperatures well above the condensation point of the hazardous compound so as to avoid generation of a mixed waste residue.
Present particulate containment technologies include filtration, scrubbing, electrostatic precipitation, and cyclonic separators. These technologies represent a relatively high cost of installation and operation or simply cannot provide the performance required for specific applications.
Cyclonic separators operate at high temperature, which desirably allows the high temperature flue gas to be used in downstream equipment. However, such separators only separate particles with diameters greater than 5 micrometers. Furthermore, cyclonic separators are generally not efficient compared to other separator technologies. In a cyclonic separator, gas is forced to spin in a containment vessel and it is by means of centrifugal force that particles are spun outward to an outer wall where they drop to a collection hopper. The centrifugal force which can be reasonably generated is insufficient to cause particles of a diameter of less than 5 microns to spin outward to the outer wall, given the turbulent and viscous forces of the gas. Unfortunately, it has been shown that particles smaller than this, especially in the 1 micron range, are best absorbed and retained by the human pulmonary system, and thus may contribute to respiratory ailments such as bronchitis, emphysema and lung cancer.
Electrostatic precipitators came into commercial service in the early 1900s. Literally hundreds of discharge electrode configurations have been utilized. Electrostatic precipitators capture dust or fly ash by charging the individual particles and then accelerating them in an electric field until they come into contact with a grounded collecting surface. Agglomeration of the particles occurs at the collecting surface which allows the resulting agglomerates to be mechanically rapped or in some cases washed from the collecting surfaces into the hoppers below each collecting field. The efficiency of such precipitators is highly sensitive to particle resistivity. At the extremes of particle resistivity, both low and high, precipitator efficiency is negatively impacted. When particle resistivity is low, precipitator efficiency may degrade due to reentrainment of the collected dust. The combined effects of gas flow and the weight of the collected material act to dislodge and reentrain collected material in the gas flow. Further, when the collecting surfaces of conventional precipitators are rapped, some of the collected dust cake is redispersed in the gas stream as small particles.
With high particulate resistivity, a high voltage drop develops across the collected dust cake layer, resulting in the phenomenon know as "back ionization". Back ionization impedes the normal flow of negative ions, or positively discharges dust particles, ejecting collected particulates from the dust cake. In severe cases, the precipitation process essentially stops.
Unfortunately, at high temperatures, particulate resistivity generally decreases. Consequently, electrostatic precipitators cannot effectively operate at high temperatures.
Water spray scrubbers cannot operate above the steam point of 100.degree. C. Baghouse filters also cannot operate at high temperatures. Furthermore, baghouse filters are susceptible to tearing. Finally, filters and water spray scrubbers result in an increase in the volume of waste materials, because the filters and water must be collected and disposed of as waste.
In a flue gas containing sulphur oxide gases and nitrogen oxide gases, such as from a coal-fired boiler, the injection of sorbent particles and ammonia gas ahead of a high temperature fabric filter has been practiced. The sorbent particles remove the sulphur oxide gases from the flue gas and are themselves removed by the filter along with the fly ash. The filter holds a catalyst for conversion of nitrogen oxides and ammonia to nitrogen and water. The fabric filters include the high cost of continuous fiber ceramic sleeves and are susceptible to clogging by condensable vapors. The destruction or removal efficiencies achieved with the filter system are only 70% for sulphur oxides, 90% for nitrogen oxides, and 99% for particulates.
There is a need in high temperature thermal processes employed in both the power plant industry and environmental clean-up industry for improved means of reducing the particulate and hazardous waste contents of a high temperature flue gas. In particular, there is a need for an efficient treatment which effectively removes small particles and enhances the action of sorbents and catalysts typically used for removal of sulfur oxides and nitrogen oxides, which treatment is completed while the flue gas is at a high temperature, such that the flue gas may be used in downstream equipment without the abrasive effect of fly ash or the corrosive effect of acidic gases. Finally, there is a need for a treatment which will not result in an increased volume of hazardous waste which must be disposed of.